Using KUnit

The purpose of this document is to describe what KUnit is, how it works, how it is intended to be used, and all the concepts and terminology that are needed to understand it. This guide assumes a working knowledge of the Linux kernel and some basic knowledge of testing.

For a high level introduction to KUnit, including setting up KUnit for your project, see Getting Started.

Organization of this document

This document is organized into two main sections: Testing and Isolating Behavior. The first covers what a unit test is and how to use KUnit to write them. The second covers how to use KUnit to isolate code and make it possible to unit test code that was otherwise un-unit-testable.

Testing

What is KUnit?

“K” is short for “kernel” so “KUnit” is the “(Linux) Kernel Unit Testing Framework.” KUnit is intended first and foremost for writing unit tests; it is general enough that it can be used to write integration tests; however, this is a secondary goal. KUnit has no ambition of being the only testing framework for the kernel; for example, it does not intend to be an end-to-end testing framework.

What is Unit Testing?

A unit test is a test that tests code at the smallest possible scope, a unit of code. In the C programming language that’s a function.

Unit tests should be written for all the publicly exposed functions in a compilation unit; so that is all the functions that are exported in either a class (defined below) or all functions which are not static.

Writing Tests

Test Cases

The fundamental unit in KUnit is the test case. A test case is a function with the signature void (*)(struct kunit *test). It calls a function to be tested and then sets expectations for what should happen. For example:

void example_test_success(struct kunit *test)
{
}

void example_test_failure(struct kunit *test)
{
        KUNIT_FAIL(test, "This test never passes.");
}

In the above example example_test_success always passes because it does nothing; no expectations are set, so all expectations pass. On the other hand example_test_failure always fails because it calls KUNIT_FAIL, which is a special expectation that logs a message and causes the test case to fail.

Expectations

An expectation is a way to specify that you expect a piece of code to do something in a test. An expectation is called like a function. A test is made by setting expectations about the behavior of a piece of code under test; when one or more of the expectations fail, the test case fails and information about the failure is logged. For example:

void add_test_basic(struct kunit *test)
{
        KUNIT_EXPECT_EQ(test, 1, add(1, 0));
        KUNIT_EXPECT_EQ(test, 2, add(1, 1));
}

In the above example add_test_basic makes a number of assertions about the behavior of a function called add; the first parameter is always of type struct kunit *, which contains information about the current test context; the second parameter, in this case, is what the value is expected to be; the last value is what the value actually is. If add passes all of these expectations, the test case, add_test_basic will pass; if any one of these expectations fail, the test case will fail.

It is important to understand that a test case fails when any expectation is violated; however, the test will continue running, potentially trying other expectations until the test case ends or is otherwise terminated. This is as opposed to assertions which are discussed later.

To learn about more expectations supported by KUnit, see Test API.

Note

A single test case should be pretty short, pretty easy to understand, focused on a single behavior.

For example, if we wanted to properly test the add function above, we would create additional tests cases which would each test a different property that an add function should have like this:

void add_test_basic(struct kunit *test)
{
        KUNIT_EXPECT_EQ(test, 1, add(1, 0));
        KUNIT_EXPECT_EQ(test, 2, add(1, 1));
}

void add_test_negative(struct kunit *test)
{
        KUNIT_EXPECT_EQ(test, 0, add(-1, 1));
}

void add_test_max(struct kunit *test)
{
        KUNIT_EXPECT_EQ(test, INT_MAX, add(0, INT_MAX));
        KUNIT_EXPECT_EQ(test, -1, add(INT_MAX, INT_MIN));
}

void add_test_overflow(struct kunit *test)
{
        KUNIT_EXPECT_EQ(test, INT_MIN, add(INT_MAX, 1));
}

Notice how it is immediately obvious what all the properties that we are testing for are.

Assertions

KUnit also has the concept of an assertion. An assertion is just like an expectation except the assertion immediately terminates the test case if it is not satisfied.

For example:

static void mock_test_do_expect_default_return(struct kunit *test)
{
        struct mock_test_context *ctx = test->priv;
        struct mock *mock = ctx->mock;
        int param0 = 5, param1 = -5;
        const char *two_param_types[] = {"int", "int"};
        const void *two_params[] = {&param0, &param1};
        const void *ret;

        ret = mock->do_expect(mock,
                              "test_printk", test_printk,
                              two_param_types, two_params,
                              ARRAY_SIZE(two_params));
        KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ret);
        KUNIT_EXPECT_EQ(test, -4, *((int *) ret));
}

In this example, the method under test should return a pointer to a value, so if the pointer returned by the method is null or an errno, we don’t want to bother continuing the test since the following expectation could crash the test case. ASSERT_NOT_ERR_OR_NULL(…) allows us to bail out of the test case if the appropriate conditions have not been satisfied to complete the test.

Test Suites

Now obviously one unit test isn’t very helpful; the power comes from having many test cases covering all of your behaviors. Consequently it is common to have many similar tests; in order to reduce duplication in these closely related tests most unit testing frameworks provide the concept of a test suite, in KUnit we call it a test suite; all it is is just a collection of test cases for a unit of code with a set up function that gets invoked before every test cases and then a tear down function that gets invoked after every test case completes.

Example:

static struct kunit_case example_test_cases[] = {
        KUNIT_CASE(example_test_foo),
        KUNIT_CASE(example_test_bar),
        KUNIT_CASE(example_test_baz),
        {}
};

static struct kunit_suite example_test_suite = {
        .name = "example",
        .init = example_test_init,
        .exit = example_test_exit,
        .test_cases = example_test_cases,
};
kunit_test_suite(example_test_suite);

In the above example the test suite, example_test_suite, would run the test cases example_test_foo, example_test_bar, and example_test_baz, each would have example_test_init called immediately before it and would have example_test_exit called immediately after it. kunit_test_suite(example_test_suite) registers the test suite with the KUnit test framework.

Note

A test case will only be run if it is associated with a test suite.

For a more information on these types of things see the Test API.

Isolating Behavior

The most important aspect of unit testing that other forms of testing do not provide is the ability to limit the amount of code under test to a single unit. In practice, this is only possible by being able to control what code gets run when the unit under test calls a function and this is usually accomplished through some sort of indirection where a function is exposed as part of an API such that the definition of that function can be changed without affecting the rest of the code base. In the kernel this primarily comes from two constructs, classes, structs that contain function pointers that are provided by the implementer, and architecture specific functions which have definitions selected at compile time.

Classes

Classes are not a construct that is built into the C programming language; however, it is an easily derived concept. Accordingly, pretty much every project that does not use a standardized object oriented library (like GNOME’s GObject) has their own slightly different way of doing object oriented programming; the Linux kernel is no exception.

The central concept in kernel object oriented programming is the class. In the kernel, a class is a struct that contains function pointers. This creates a contract between implementers and users since it forces them to use the same function signature without having to call the function directly. In order for it to truly be a class, the function pointers must specify that a pointer to the class, known as a class handle, be one of the parameters; this makes it possible for the member functions (also known as methods) to have access to member variables (more commonly known as fields) allowing the same implementation to have multiple instances.

Typically a class can be overridden by child classes by embedding the parent class in the child class. Then when a method provided by the child class is called, the child implementation knows that the pointer passed to it is of a parent contained within the child; because of this, the child can compute the pointer to itself because the pointer to the parent is always a fixed offset from the pointer to the child; this offset is the offset of the parent contained in the child struct. For example:

struct shape {
        int (*area)(struct shape *this);
};

struct rectangle {
        struct shape parent;
        int length;
        int width;
};

int rectangle_area(struct shape *this)
{
        struct rectangle *self = container_of(this, struct shape, parent);

        return self->length * self->width;
};

void rectangle_new(struct rectangle *self, int length, int width)
{
        self->parent.area = rectangle_area;
        self->length = length;
        self->width = width;
}

In this example (as in most kernel code) the operation of computing the pointer to the child from the pointer to the parent is done by container_of.

Faking Classes

In order to unit test a piece of code that calls a method in a class, the behavior of the method must be controllable, otherwise the test ceases to be a unit test and becomes an integration test.

A fake just provides an implementation of a piece of code that is different than what runs in a production instance, but behaves identically from the standpoint of the callers; this is usually done to replace a dependency that is hard to deal with, or is slow.

A good example for this might be implementing a fake EEPROM that just stores the “contents” in an internal buffer. For example, let’s assume we have a class that represents an EEPROM:

struct eeprom {
        ssize_t (*read)(struct eeprom *this, size_t offset, char *buffer, size_t count);
        ssize_t (*write)(struct eeprom *this, size_t offset, const char *buffer, size_t count);
};

And we want to test some code that buffers writes to the EEPROM:

struct eeprom_buffer {
        ssize_t (*write)(struct eeprom_buffer *this, const char *buffer, size_t count);
        int flush(struct eeprom_buffer *this);
        size_t flush_count; /* Flushes when buffer exceeds flush_count. */
};

struct eeprom_buffer *new_eeprom_buffer(struct eeprom *eeprom);
void destroy_eeprom_buffer(struct eeprom *eeprom);

We can easily test this code by faking out the underlying EEPROM:

struct fake_eeprom {
        struct eeprom parent;
        char contents[FAKE_EEPROM_CONTENTS_SIZE];
};

ssize_t fake_eeprom_read(struct eeprom *parent, size_t offset, char *buffer, size_t count)
{
        struct fake_eeprom *this = container_of(parent, struct fake_eeprom, parent);

        count = min(count, FAKE_EEPROM_CONTENTS_SIZE - offset);
        memcpy(buffer, this->contents + offset, count);

        return count;
}

ssize_t fake_eeprom_write(struct eeprom *this, size_t offset, const char *buffer, size_t count)
{
        struct fake_eeprom *this = container_of(parent, struct fake_eeprom, parent);

        count = min(count, FAKE_EEPROM_CONTENTS_SIZE - offset);
        memcpy(this->contents + offset, buffer, count);

        return count;
}

void fake_eeprom_init(struct fake_eeprom *this)
{
        this->parent.read = fake_eeprom_read;
        this->parent.write = fake_eeprom_write;
        memset(this->contents, 0, FAKE_EEPROM_CONTENTS_SIZE);
}

We can now use it to test struct eeprom_buffer:

struct eeprom_buffer_test {
        struct fake_eeprom *fake_eeprom;
        struct eeprom_buffer *eeprom_buffer;
};

static void eeprom_buffer_test_does_not_write_until_flush(struct kunit *test)
{
        struct eeprom_buffer_test *ctx = test->priv;
        struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
        struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
        char buffer[] = {0xff};

        eeprom_buffer->flush_count = SIZE_MAX;

        eeprom_buffer->write(eeprom_buffer, buffer, 1);
        KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);

        eeprom_buffer->write(eeprom_buffer, buffer, 1);
        KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0);

        eeprom_buffer->flush(eeprom_buffer);
        KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
        KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
}

static void eeprom_buffer_test_flushes_after_flush_count_met(struct kunit *test)
{
        struct eeprom_buffer_test *ctx = test->priv;
        struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
        struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
        char buffer[] = {0xff};

        eeprom_buffer->flush_count = 2;

        eeprom_buffer->write(eeprom_buffer, buffer, 1);
        KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);

        eeprom_buffer->write(eeprom_buffer, buffer, 1);
        KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
        KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
}

static void eeprom_buffer_test_flushes_increments_of_flush_count(struct kunit *test)
{
        struct eeprom_buffer_test *ctx = test->priv;
        struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
        struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
        char buffer[] = {0xff, 0xff};

        eeprom_buffer->flush_count = 2;

        eeprom_buffer->write(eeprom_buffer, buffer, 1);
        KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);

        eeprom_buffer->write(eeprom_buffer, buffer, 2);
        KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
        KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
        /* Should have only flushed the first two bytes. */
        KUNIT_EXPECT_EQ(test, fake_eeprom->contents[2], 0);
}

static int eeprom_buffer_test_init(struct kunit *test)
{
        struct eeprom_buffer_test *ctx;

        ctx = kunit_kzalloc(test, sizeof(*ctx), GFP_KERNEL);
        KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx);

        ctx->fake_eeprom = kunit_kzalloc(test, sizeof(*ctx->fake_eeprom), GFP_KERNEL);
        KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx->fake_eeprom);
        fake_eeprom_init(ctx->fake_eeprom);

        ctx->eeprom_buffer = new_eeprom_buffer(&ctx->fake_eeprom->parent);
        KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx->eeprom_buffer);

        test->priv = ctx;

        return 0;
}

static void eeprom_buffer_test_exit(struct kunit *test)
{
        struct eeprom_buffer_test *ctx = test->priv;

        destroy_eeprom_buffer(ctx->eeprom_buffer);
}

KUnit on non-UML architectures

By default KUnit uses UML as a way to provide dependencies for code under test. Under most circumstances KUnit’s usage of UML should be treated as an implementation detail of how KUnit works under the hood. Nevertheless, there are instances where being able to run architecture specific code, or test against real hardware is desirable. For these reasons KUnit supports running on other architectures.

Running existing KUnit tests on non-UML architectures

There are some special considerations when running existing KUnit tests on non-UML architectures:

  • Hardware may not be deterministic, so a test that always passes or fails when run under UML may not always do so on real hardware.
  • Hardware and VM environments may not be hermetic. KUnit tries its best to provide a hermetic environment to run tests; however, it cannot manage state that it doesn’t know about outside of the kernel. Consequently, tests that may be hermetic on UML may not be hermetic on other architectures.
  • Some features and tooling may not be supported outside of UML.
  • Hardware and VMs are slower than UML.

None of these are reasons not to run your KUnit tests on real hardware; they are only things to be aware of when doing so.

The biggest impediment will likely be that certain KUnit features and infrastructure may not support your target environment. For example, at this time the KUnit Wrapper (tools/testing/kunit/kunit.py) does not work outside of UML. Unfortunately, there is no way around this. Using UML (or even just a particular architecture) allows us to make a lot of assumptions that make it possible to do things which might otherwise be impossible.

Nevertheless, all core KUnit framework features are fully supported on all architectures, and using them is straightforward: all you need to do is to take your kunitconfig, your Kconfig options for the tests you would like to run, and merge them into whatever config your are using for your platform. That’s it!

For example, let’s say you have the following kunitconfig:

CONFIG_KUNIT=y
CONFIG_KUNIT_EXAMPLE_TEST=y

If you wanted to run this test on an x86 VM, you might add the following config options to your .config:

CONFIG_KUNIT=y
CONFIG_KUNIT_EXAMPLE_TEST=y
CONFIG_SERIAL_8250=y
CONFIG_SERIAL_8250_CONSOLE=y

All these new options do is enable support for a common serial console needed for logging.

Next, you could build a kernel with these tests as follows:

make ARCH=x86 olddefconfig
make ARCH=x86

Once you have built a kernel, you could run it on QEMU as follows:

qemu-system-x86_64 -enable-kvm \
                   -m 1024 \
                   -kernel arch/x86_64/boot/bzImage \
                   -append 'console=ttyS0' \
                   --nographic

Interspersed in the kernel logs you might see the following:

TAP version 14
        # Subtest: example
        1..1
        # example_simple_test: initializing
        ok 1 - example_simple_test
ok 1 - example

Congratulations, you just ran a KUnit test on the x86 architecture!

Writing new tests for other architectures

The first thing you must do is ask yourself whether it is necessary to write a KUnit test for a specific architecture, and then whether it is necessary to write that test for a particular piece of hardware. In general, writing a test that depends on having access to a particular piece of hardware or software (not included in the Linux source repo) should be avoided at all costs.

Even if you only ever plan on running your KUnit test on your hardware configuration, other people may want to run your tests and may not have access to your hardware. If you write your test to run on UML, then anyone can run your tests without knowing anything about your particular setup, and you can still run your tests on your hardware setup just by compiling for your architecture.

Important

Always prefer tests that run on UML to tests that only run under a particular architecture, and always prefer tests that run under QEMU or another easy (and monitarily free) to obtain software environment to a specific piece of hardware.

Nevertheless, there are still valid reasons to write an architecture or hardware specific test: for example, you might want to test some code that really belongs in arch/some-arch/*. Even so, try your best to write the test so that it does not depend on physical hardware: if some of your test cases don’t need the hardware, only require the hardware for tests that actually need it.

Now that you have narrowed down exactly what bits are hardware specific, the actual procedure for writing and running the tests is pretty much the same as writing normal KUnit tests. One special caveat is that you have to reset hardware state in between test cases; if this is not possible, you may only be able to run one test case per invocation.